Do you routinely ask someone to do something that isn’t related to their job because you know they’ll say “yes”? If so, you could be on your way to losing your best staff.
The problem with asking a “yes” person to do extra tasks, like coordinating a staff event or leading an activity, is that they may believe these things will help them in their career. When they find out that stepping up isn’t leading to a promotion, they’ll be extremely disappointed and inclined to leave. So, while it’s good for managers in the short-term to have “yes” people, if the extra tasks don’t lead to people developing or progressing in their role, you’re going to find yourself losing talent. Never imply that ‘doing more’ is going to be good for an employee’s career, if it’s not. Be clear from the start and you won’t regret it later.